Question

I have to avoid page refresh. Every time I click the submit button, page is refreshed. How do I avoid that?

protected void Button1_Click(object sender, EventArgs e)
{
    string firstname = DropDownList1.SelectedItem.Text;

    if (firstname == "All")
    {
        da = new SqlDataAdapter(query, con);
    }

    dt = new DataTable();
    dt = ds.DataTable1;
    da.Fill(dt);
    rdc.Load(Server.MapPath("CrystalReport.rpt"));
    rdc.SetDataSource(ds);
    CrystalReportViewer1.ReportSource = rdc;
    CrystalReportViewer1.RefreshReport();
}

Answer 1

I would suggest putting everything that needs to be updated in an <asp:UpdatePanel>.
You can find that control in the Ajax Control Toolkit

You will also have to add a <asp:ScriptManager> on top of your page.